The invention relates to an optical arrangement, in particular a projection exposure system for microlithography, in particular having slit-shaped illumination, having at least one light source that emits radiation and an optical element that is heated by exposure to the radiation, wherein the optical element is exposed to the emitted radiation of the light source with non-rotational-symmetric intensity distribution.
The imaging quality of such an optical arrangement is often reduced by non-rotational-symmetric imaging errors. Such imaging errors arise, for example, not only as a result of non-rotational-symmetric light-induced heating of the optical element, but also as a result of other light-induced effects, such as, for example, xe2x80x9ccompactionxe2x80x9d, that result in a corresponding non-rotation-symmetrical expansion or refractive-index distribution in the optical element.
Given high requirements imposed on the imaging quality, such as those demanded, in particular, in projection exposure methods in microlithography, the light-induced imaging errors described cannot be tolerated.
From the generic EP 0 678 768 A2, it is known to strive for an improvement in the imaging properties by symmetrizing or homogenizing the temperature distribution by an additional heating. In this case, the additional heating takes place actively by means of a plurality of heating elements that are thermally coupled to the circumferential area of a lens. Such heating of the lens has the disadvantage that the circumferential area of the lens has to be heated relatively strongly to achieve, despite the thermal conduction properties of the lens material, which are as a rule poor, the desired symmetrization or homogenization of the temperature distribution in the central region of the lens, which is most relevant for the imaging properties. A strong heating of the circumferential region of the lens results, however, in a risk of damage to the lens and/or the lens mounting as a result of thermal stresses.
Because of the relatively large spacing between the circumferential region and the central region, which is modified by the radiation of a light source serving for projection, a controlled structured modifying of the temperature distribution in the vicinity of the central region is, in addition virtually scarcely possible through heating the circumferential region.
In the likewise generic EP 0 823 662 A2, optical elements are additionally heated in order to compensate for the projection light-induced imaging errors by coupling in additional light sources that heat the optical elements by absorption at those points where they are not irradiated by the projection light. Since the heating power due to the additional light sources have to be comparable to those of the projection light source to symmetrize or homogenize the temperature distribution, for the additional light sources, either light powers of the order of magnitude of those of the projection light source are necessary or the additional light sources must operate in wavelength ranges that are more heavily absorbed by the material of the optical elements. In both cases, expensive additional light sources are necessary. In the first case, light sources having high power are necessary, and in the second case, light sources having a wavelength are necessary that are not available at low cost.
The object of a the present invention is therefore to develop an optical arrangement of the type mentioned at the outset in such a way that a better symmetrization or homogenization of the temperature distribution in the optical element can be achieved by simple means.
This object is achieved, according to the invention, in that
a) the optical element has an absorbing coating having a spatial distribution such that
b) the absorption of the coating is non-rotational-symmetric in an at least approximately complementary manner to the intensity distribution of the exposure to the radiation of the light source.
Such an absorbing coating results in an increased flexibility if a specified additional heating of the lens is to be established by means of the light absorption. New degrees of freedom, such as the absorption coefficient and the form of the absorbing coating, are then available for modifying the distribution of the additional heating.
Preferably, the absorbing coating comprises at least two portions between which there remains a non-absorbingly coated region whose dimension transversely to the axis of the radiation is smaller in at least one direction than the cross section of the radiation measured in the same direction. As a result, the additional heating can be brought about by means of the absorption of the projection light itself. For this purpose, a smaller proportion of the projection light is absorbed in the coating and results in the desired symmetrization or homogenization of the temperature distribution in the optical element by means of thermal conduction in the coating and thermal coupling to the optical element. The proportion of projection light that is absorbed in the coating is in this case so small that the projection quality is virtually unaffected thereby.
The absorbing coating has an absorptive power, varying over its surface, for wavelengths of the radiation of the light source. As a result, a more substantial adjustment of the spatial distribution of the additional heating is possible by means of the light absorption in the coating in order to symmetrize or homogenize the entire temperature distribution of the optical element.
Such an absorptive power varying over the surface can be achieved, for example, as follows:
The absorbing coating may vary in its layer thickness. This makes possible coating with uniform material whose absorptive power nevertheless varies over the surface.
Alternatively or additionally, the absorbing coating varies spatially in its absorption coefficient. As a result, either a coating is possible that has an absorptive power varying over the surface with constant layer thickness, which is advantageous for the production of anti-reflection layers, or an additional degree of freedom is provided for the production of coatings varying in absorption over their surface. In the latter case, relatively complex absorption structures can also be produced. Such a variation in the absorption coefficient can be achieved, for example, by controlled doping of the absorption coating.
Preferably, the absorbing coating has the highest absorptive power in the region that is nearest the centre of the surface of the optical element exposed to the radiation. Additional heating necessary for the symmetrization or homogenization then takes place in the vicinity of the central region of the optical element, which is the most strongly heated by the projection light, so that at that point a strong additional heating can take place in a controlled manner for the purpose of symmetrization or homogenization. An unnecessary heating of more remote regions in order additionally to heat the region in the vicinity of the central region heated by the projection light by thermal conduction is superfluous.
In a refinement of the invention, the light source has a projection light source and a compensating light source, wherein the radiation of the compensating light source is directed at the absorbing coating. The absorption coefficient of the coating and the compensating light source can be matched to one another in such a way that compensating light sources having very low light powers and a standard emission wavelength can be used that are correspondingly less expensive and less complicated than in the subject matter of the abovementioned EP 0 823 662 A2. Thus, the absorption coefficient of the coating can be adapted, for example, to the emission wavelength of the known inexpensive light sources, such as laser diodes. An additional control of the temperature distribution is possible via the surfaces of the optical element exposed to compensating light and via the distribution of the light power over the exposed surfaces. Optionally, the additional heating can also be adapted to the requirements by adjusting the size and the position of the surface exposed to the compensating light. This is done by suitable alignment of the compensating light sources.
The spatial distribution of the absorbing coating may be such that optical image errors of at least one other optical element are compensated for by the non-rotational-symmetric absorption of the radiation of the light source in an optical element. Such an overcompensation makes possible compensation for imaging errors of a system of optical elements. Under these circumstances, only one of said optical elements has to have an absorbing coating.